The invention generally relates to electrochemical cells.
Batteries are commonly used electrical energy sources. A battery contains a negative electrode, typically called the anode, and a positive electrode, typically called the cathode. The anode contains an active material that can be oxidized; the cathode contains or consumes an active material that can be reduced. The anode active material is capable of reducing the cathode active material. In order to prevent direct reaction of the anode material and the cathode material, the anode and the cathode are electrically isolated from each other by a sheet-like layer, typically called the separator.
When a battery is used as an electrical energy source in a device, such as a hearing aid or a cellular telephone, electrical contact is made to the anode and the cathode, allowing electrons to flow through the device and permitting the respective oxidation and reduction reactions to occur to provide electrical power. An electrolyte in contact with the anode and the cathode contains ions that flow through the separator between the electrodes to maintain charge balance throughout the battery during discharge.
In a metal air electrochemical cell, the cathode structure contains a material that can catalyze the reduction of oxygen which enters the cell as a component of atmospheric air passing through access ports in the container. Zinc oxide, or zincate, is formed in the anode. Thus, the overall electrochemical reaction within the cell results in zinc metal being oxidized to zinc ions and oxygen from the air being reduced to hydroxyl ions. While these chemical reactions are taking place, electrons are transferred from the anode to the cathode, providing power to the device.
Zinc cathode materials have traditionally been manufactured using flat processes such as dry coating and paste coating. For use in cylindrical zinc air batteries, the flat cathode sheets are welded together to create a cathode tube. Cathode tubes made by such processes typically have a seam were the edges of the sheet are welded together to form the cylindrical structure.
In general, the invention relates to cathode tubes for metal air electrochemical cells and methods for making the cathode tubes. The cathode tubes are seamless. Unlike cathode tubes made by flat processed materials which have reduced ionic conversion at the seam, the seamless cathode tubes allow for the total surface of the tube to be utilized for conversion of hydroxyl ions. In addition, production of a seamless cathode tube eliminates the potential for leakage at the seam of the tube. As a result, electrochemical cells including these seamless cathode tubes can therefore be used to produce seamless cathode tubes for use in electrochemical cells which have good discharge characteristics and leakage resistance.
In one aspect, the invention features a method of making a seamless cathode tube. The method includes applying a cathode coating mix to a current collector tube, the cathode coating mix stiffening to form a seamless cathode tube.
Preferably, the cathode tube is made by applying a coating mix which includes a binder, e.g., a polymer, which is hydrophobic and/or has a fibrous final state. Examples of such binders include polytetrafluoroethylene (PTFE), PTFE copolymers, polyvinylidenefluoride (PVDF), hexofluoropyopalene (HFP)/ polyvinylidenefluoride (PVDF) copolymers, HFP/PVDF/PTFE tetrapolymers, polyethylene (PE) and ultrahydrated PE. The coating mix can be applied onto the current collector tube, e.g., a screen tube, such that a binder such as PTFE fibrillates and the cathode coating mix stiffens to form a seamless cathode tube. Methods of applying the coating mix such that the binder fibrillates include continuous extrusion, impact extrusion, compression molding, transfer molding and injection molding. These methods push binder particles past each other resulting in stretching and fiber formation, i.e., fibrillation. In addition, these methods induce fibrillation later in the processing stage, e.g., as the coating mix is being applied to the current collector, thereby avoiding overstretching of the binder fibers which can occur if fibrillation happens too early in the process.
There are several advantages to using a binder such as PTFE. PTFE is hydrophobic and resists wetting, thereby creating pathways for oxygen to enter the cathode structure. In addition, due to its very high molecular weight, PTFE has inherent fibrillation properties. The particle to particle contact of fibrillation results in voids. Thus, a binder such as PTFE provides porosity to the finished cathode tube. PTFE also provides structural integrity and flexibility to the finished cathode tube.
The cathode coating mix can also include a solvent. The addition of a solvent to the coating mix has several advantages. For example, the addition of a solvent to the coating mix forms a flowable mixture that can be applied to the current collector tube using extrusion, e.g., continuous or impact extrusion, or molding, e.g., compression, transfer, or injection molding. In addition, the solvent can serve as a lubricant to control fibrillation of the binder, thereby decreasing the amount of fibrillation that occurs prior to application of the mix to the current collector tube. The solvent can later be removed from the cathode tube during a lubricant/solvent removal step.
In another aspect, the invention features a method of making a seamless cathode tube by injection molding. The method can include placing a current collector tube into a molding cavity, and injecting a cathode coating mix. The cathode coating mix stiffens to form a seamless cathode tube. The method can also include injection molding the cathode mix into a tube.
The production of seamless cathode tubes by injection molding results in a more efficient assembly process which can be used to produce discrete finished parts. The injection molding process, as compared to converting flat sheet material, results in fewer process steps and components to complete the cathode sub assembly and subassembly of the electrochemical cell itself. The production of seamless cathode tubes by injection molding provides a more homogenous structure that flat sheets that are converted into tubes. The methods of making seamless cathode tubes using injection molding can therefore be used to make seamless cathode tubes having good performance characteristics made in a high rate assembly process.
Other features and advantages of the invention will be apparent from the description of the preferred embodiment thereof and from the claims.